NO. 3 RADIATION OF THE ATMOSPHERE — ANGSTROM 83 



means by which heat is carried away from the surface, evaporation, 

 and especially convection, being factors that are not negligible. It 

 is probable that if a part of the solar radiation is really absorbed by 

 the volcanic dust, this will tend to diminish the temperature gradient 

 between the sea level and the upper strata of the atmosphere, and 

 consequently cause a decrease in the vertical heat convection from 

 the lower stations. A second access of radiation is due to the scattered 

 skylight, and Abbot as well as Dorno point out that the sum of sky- 

 light and direct solar radiation was subjected to only a relatively small 

 change by the effect of the dust. One has naturally to expect that if 

 a part of the direct solar radiation is uniformly scattered by the atmos- 

 phere, a part of the scattered radiation will reach the surface of the 

 earth in the form of skylight, this part increasing with an increase 

 in the scattering power. Part of the scattered radiation is reflected 

 out to space. Similar conditions naturally hold for the nocturnal 

 radiation, and it is evident that the quantity measured by the instru- 

 ment will always be the outgoing heat radiation diminished by the 

 part of this radiation that is reflected back by the diffusing atmos- 

 phere upon the radiating surface.' 



C. RADIATION FROM LARGE WATER SURFACES 



The radiation from bodies with reflecting but not absorbing or 

 diffusing surfaces depends upon their reflecting power and their 

 temperature only. The emission of radiation in a direction that 

 makes an angle <j> with the normal to the surface at the point con- 

 sidered, is determined by the relation : 



E 4> = e <p (i-R 4> ) 



where <-0 is the radiation of a black surface in the direction </>, and 

 R^ the reflected fraction of the light incident in the named direction. 

 For the total radiation emitted we have 



E (t> = jf0(i— Rti>)dQ, 



where the integration is to be extended over the whole hemisphere. 

 In chapter VI, I have given an account of some observations that 

 show in what way the radiation from a black surface to the sky is 

 dependent on the direction. As a very large part of the earth's sur- 

 face is covered with water, and therefore slightly different from the 

 conditions defined by the " black surface," I have thought it to be of 

 interest to give here a brief discussion of the case where we have, 

 instead of the black surface, a plane water surface radiating out to 



